There could be a couple of reasons why they didn't spot it:
1. The creature is coloured fairly similarly to the ground around it so when travelling fairly fast on horseback you wouldn't spot it.
2. The photographer was focusing on the horseriders and not the creature.
3. The alien has a stealth device that cloaks it from human view but allows a camera to take photos of it!?!?!?!
this could work if it works on human perception and not changing the way light travels